The primary focus of the past year's research has been on the modulatory role of peripheral hormonal systems in influencing learning. Initially we characterized the actions of the opioid agonists Leu-enkephalin and Met-enkephalin on a discriminated Y-maze escape task in mice. Leu-enkephalin was found to impair and Met-enkephalin to enhance acquisition of the response at equivalent doses (100 micrograms/kg, i.p.). Naloxone (1.0 and 10.0 mg/kg) also enhanced acquisition of the escape response, and blocked the impairing actions of Leu-enkephalin. Neither naloxone nor the enkephalins influenced shock-induced locomotor activity in an open field. The results suggest that enkephalin actions on escape conditioning are mediated through opioid receptors. In the final series of Leu-enkephalin studies conducted this year it was found that this peptide produces long term effects on one-way active avoidance conditioning in the mouse.